ref: c6d4cd09a395a24ee993c1edf1031e5b01adddde
dir: /sys/src/libthread/channel.c/
#include <u.h>
#include <libc.h>
#include <thread.h>
#include "threadimpl.h"
/* Value to indicate the channel is closed */
enum {
CHANCLOSD = 0xc105ed,
};
static char errcl[] = "channel was closed";
static Lock chanlock; /* central channel access lock */
static void enqueue(Alt*, Channel**);
static void dequeue(Alt*);
static int canexec(Alt*);
static int altexec(Alt*, int);
#define Closed ((void*)CHANCLOSD)
#define Intred ((void*)~0) /* interrupted */
static void
_chanfree(Channel *c)
{
int i, inuse;
if(c->closed == 1) /* chanclose is ongoing */
inuse = 1;
else{
inuse = 0;
for(i = 0; i < c->nentry; i++) /* alt ongoing */
if(c->qentry[i])
inuse = 1;
}
if(inuse)
c->freed = 1;
else{
if(c->qentry)
free(c->qentry);
free(c);
}
}
void
chanfree(Channel *c)
{
lock(&chanlock);
_chanfree(c);
unlock(&chanlock);
}
Channel*
chancreate(int elemsize, int elemcnt)
{
Channel *c;
if(elemcnt < 0 || elemsize <= 0)
return nil;
c = _threadmalloc(sizeof(Channel)+elemsize*elemcnt, 1);
c->e = elemsize;
c->s = elemcnt;
_threaddebug(DBGCHAN, "chancreate %p", c);
return c;
}
static int
isopenfor(Channel *c, int op)
{
return c->closed == 0 || (op == CHANRCV && c->n > 0);
}
int
alt(Alt *alts)
{
Alt *a, *xa, *ca;
Channel volatile *c;
int n, s, waiting, allreadycl;
void* r;
Thread *t;
/*
* The point of going splhi here is that note handlers
* might reasonably want to use channel operations,
* but that will hang if the note comes while we hold the
* chanlock. Instead, we delay the note until we've dropped
* the lock.
*/
t = _threadgetproc()->thread;
if(t->moribund || _threadexitsallstatus)
yield(); /* won't return */
s = _procsplhi();
lock(&chanlock);
t->alt = alts;
t->chan = Chanalt;
/* test whether any channels can proceed */
n = 0;
a = nil;
for(xa=alts; xa->op!=CHANEND && xa->op!=CHANNOBLK; xa++){
xa->entryno = -1;
if(xa->op == CHANNOP)
continue;
c = xa->c;
if(c==nil){
unlock(&chanlock);
_procsplx(s);
t->chan = Channone;
return -1;
}
if(isopenfor(c, xa->op) && canexec(xa))
if(nrand(++n) == 0)
a = xa;
}
if(a==nil){
/* nothing can proceed */
if(xa->op == CHANNOBLK){
unlock(&chanlock);
_procsplx(s);
t->chan = Channone;
if(xa->op == CHANNOBLK)
return xa - alts;
}
/* enqueue on all channels open for us. */
c = nil;
ca = nil;
waiting = 0;
allreadycl = 0;
for(xa=alts; xa->op!=CHANEND; xa++)
if(xa->op==CHANNOP)
continue;
else if(isopenfor(xa->c, xa->op)){
waiting = 1;
enqueue(xa, &c);
} else if(xa->err != errcl)
ca = xa;
else
allreadycl = 1;
if(waiting == 0)
if(ca != nil){
/* everything was closed, select last channel */
ca->err = errcl;
unlock(&chanlock);
_procsplx(s);
t->chan = Channone;
return ca - alts;
} else if(allreadycl){
/* everything was already closed */
unlock(&chanlock);
_procsplx(s);
t->chan = Channone;
return -1;
}
/*
* wait for successful rendezvous.
* we can't just give up if the rendezvous
* is interrupted -- someone else might come
* along and try to rendezvous with us, so
* we need to be here.
* if the channel was closed, the op is done
* and we flag an error for the entry.
*/
Again:
unlock(&chanlock);
_procsplx(s);
r = _threadrendezvous(&c, 0);
s = _procsplhi();
lock(&chanlock);
if(r==Intred){ /* interrupted */
if(c!=nil) /* someone will meet us; go back */
goto Again;
c = (Channel*)~0; /* so no one tries to meet us */
}
/* dequeue from channels, find selected one */
a = nil;
for(xa=alts; xa->op!=CHANEND; xa++){
if(xa->op==CHANNOP)
continue;
if(xa->c == c){
a = xa;
a->err = nil;
if(r == Closed)
a->err = errcl;
}
dequeue(xa);
}
unlock(&chanlock);
_procsplx(s);
if(a == nil){ /* we were interrupted */
assert(c==(Channel*)~0);
return -1;
}
}else
altexec(a, s); /* unlocks chanlock, does splx */
_sched();
t->chan = Channone;
return a - alts;
}
int
chanclose(Channel *c)
{
Alt *a;
int i, s;
s = _procsplhi(); /* note handlers; see :/^alt */
lock(&chanlock);
if(c->closed){
/* Already close; we fail but it's ok. don't print */
unlock(&chanlock);
_procsplx(s);
return -1;
}
c->closed = 1; /* Being closed */
/*
* Locate entries that will fail due to close
* (send, and receive if nothing buffered) and wake them up.
* the situation cannot change because all queries
* should be committed by now and new ones will find the channel
* closed. We still need to take the lock during the iteration
* because we can wake threads on qentrys we have not seen yet
* as in alt and there would be a race in the access to *a.
*/
for(i = 0; i < c->nentry; i++){
if((a = c->qentry[i]) == nil || *a->tag != nil)
continue;
if(a->op != CHANSND && (a->op != CHANRCV || c->n != 0))
continue;
*a->tag = c;
unlock(&chanlock);
_procsplx(s);
while(_threadrendezvous(a->tag, Closed) == Intred)
;
s = _procsplhi();
lock(&chanlock);
}
c->closed = 2; /* Fully closed */
if(c->freed)
_chanfree(c);
unlock(&chanlock);
_procsplx(s);
return 0;
}
int
chanclosing(Channel *c)
{
int n, s;
s = _procsplhi(); /* note handlers; see :/^alt */
lock(&chanlock);
if(c->closed == 0)
n = -1;
else
n = c->n;
unlock(&chanlock);
_procsplx(s);
return n;
}
/*
* superseded by chanclosing
int
chanisclosed(Channel *c)
{
return chanisclosing(c) >= 0;
}
*/
static int
runop(int op, Channel *c, void *v, int nb)
{
int r;
Alt a[2];
/*
* we could do this without calling alt,
* but the only reason would be performance,
* and i'm not convinced it matters.
*/
a[0].op = op;
a[0].c = c;
a[0].v = v;
a[0].err = nil;
a[1].op = CHANEND;
if(nb)
a[1].op = CHANNOBLK;
switch(r=alt(a)){
case -1: /* interrupted */
return -1;
case 1: /* nonblocking, didn't accomplish anything */
assert(nb);
return 0;
case 0:
/*
* Okay, but return -1 if the op is done because of a close.
*/
if(a[0].err != nil)
return -1;
return 1;
default:
fprint(2, "ERROR: channel alt returned %d\n", r);
abort();
return -1;
}
}
int
recv(Channel *c, void *v)
{
return runop(CHANRCV, c, v, 0);
}
int
nbrecv(Channel *c, void *v)
{
return runop(CHANRCV, c, v, 1);
}
int
send(Channel *c, void *v)
{
return runop(CHANSND, c, v, 0);
}
int
nbsend(Channel *c, void *v)
{
return runop(CHANSND, c, v, 1);
}
static void
channelsize(Channel *c, int sz)
{
if(c->e != sz){
fprint(2, "expected channel with elements of size %d, got size %d\n",
sz, c->e);
abort();
}
}
int
sendul(Channel *c, ulong v)
{
channelsize(c, sizeof(ulong));
return send(c, &v);
}
ulong
recvul(Channel *c)
{
ulong v;
channelsize(c, sizeof(ulong));
if(recv(c, &v) < 0)
return ~0;
return v;
}
int
sendp(Channel *c, void *v)
{
channelsize(c, sizeof(void*));
return send(c, &v);
}
void*
recvp(Channel *c)
{
void *v;
channelsize(c, sizeof(void*));
if(recv(c, &v) < 0)
return nil;
return v;
}
int
nbsendul(Channel *c, ulong v)
{
channelsize(c, sizeof(ulong));
return nbsend(c, &v);
}
ulong
nbrecvul(Channel *c)
{
ulong v;
channelsize(c, sizeof(ulong));
if(nbrecv(c, &v) == 0)
return 0;
return v;
}
int
nbsendp(Channel *c, void *v)
{
channelsize(c, sizeof(void*));
return nbsend(c, &v);
}
void*
nbrecvp(Channel *c)
{
void *v;
channelsize(c, sizeof(void*));
if(nbrecv(c, &v) == 0)
return nil;
return v;
}
static int
emptyentry(Channel *c)
{
int i, extra;
assert((c->nentry==0 && c->qentry==nil) || (c->nentry && c->qentry));
for(i=0; i<c->nentry; i++)
if(c->qentry[i]==nil)
return i;
extra = 16;
c->nentry += extra;
c->qentry = realloc((void*)c->qentry, c->nentry*sizeof(c->qentry[0]));
if(c->qentry == nil)
sysfatal("realloc channel entries: %r");
memset(&c->qentry[i], 0, extra*sizeof(c->qentry[0]));
return i;
}
static void
enqueue(Alt *a, Channel **c)
{
int i;
_threaddebug(DBGCHAN, "Queuing alt %p on channel %p", a, a->c);
a->tag = c;
i = emptyentry(a->c);
a->c->qentry[i] = a;
}
static void
dequeue(Alt *a)
{
int i;
Channel *c;
c = a->c;
for(i=0; i<c->nentry; i++)
if(c->qentry[i]==a){
_threaddebug(DBGCHAN, "Dequeuing alt %p from channel %p", a, a->c);
c->qentry[i] = nil;
/* release if freed and not closing */
if(c->freed && c->closed != 1)
_chanfree(c);
return;
}
}
static int
canexec(Alt *a)
{
int i, otherop;
Channel *c;
c = a->c;
/* are there senders or receivers blocked? */
otherop = (CHANSND+CHANRCV) - a->op;
for(i=0; i<c->nentry; i++)
if(c->qentry[i] && c->qentry[i]->op==otherop && *c->qentry[i]->tag==nil){
_threaddebug(DBGCHAN, "can rendez alt %p chan %p", a, c);
return 1;
}
/* is there room in the channel? */
if((a->op==CHANSND && c->n < c->s)
|| (a->op==CHANRCV && c->n > 0)){
_threaddebug(DBGCHAN, "can buffer alt %p chan %p", a, c);
return 1;
}
return 0;
}
static void*
altexecbuffered(Alt *a, int willreplace)
{
uchar *v;
Channel *c;
c = a->c;
/* use buffered channel queue */
if(a->op==CHANRCV && c->n > 0){
_threaddebug(DBGCHAN, "buffer recv alt %p chan %p", a, c);
v = c->v + c->e*(c->f%c->s);
if(!willreplace)
c->n--;
c->f++;
return v;
}
if(a->op==CHANSND && c->n < c->s){
_threaddebug(DBGCHAN, "buffer send alt %p chan %p", a, c);
v = c->v + c->e*((c->f+c->n)%c->s);
if(!willreplace)
c->n++;
return v;
}
abort();
return nil;
}
static void
altcopy(void *dst, void *src, int sz)
{
if(dst){
if(src)
memmove(dst, src, sz);
else
memset(dst, 0, sz);
}
}
static int
altexec(Alt *a, int spl)
{
volatile Alt *b;
int i, n, otherop;
Channel *c;
void *me, *waiter, *buf;
c = a->c;
/* rendezvous with others */
otherop = (CHANSND+CHANRCV) - a->op;
n = 0;
b = nil;
me = a->v;
for(i=0; i<c->nentry; i++)
if(c->qentry[i] && c->qentry[i]->op==otherop && *c->qentry[i]->tag==nil)
if(nrand(++n) == 0)
b = c->qentry[i];
if(b != nil){
_threaddebug(DBGCHAN, "rendez %s alt %p chan %p alt %p", a->op==CHANRCV?"recv":"send", a, c, b);
waiter = b->v;
if(c->s && c->n){
/*
* if buffer is full and there are waiters
* and we're meeting a waiter,
* we must be receiving.
*
* we use the value in the channel buffer,
* copy the waiter's value into the channel buffer
* on behalf of the waiter, and then wake the waiter.
*/
if(a->op!=CHANRCV)
abort();
buf = altexecbuffered(a, 1);
altcopy(me, buf, c->e);
altcopy(buf, waiter, c->e);
}else{
if(a->op==CHANRCV)
altcopy(me, waiter, c->e);
else
altcopy(waiter, me, c->e);
}
*b->tag = c; /* commits us to rendezvous */
_threaddebug(DBGCHAN, "unlocking the chanlock");
unlock(&chanlock);
_procsplx(spl);
_threaddebug(DBGCHAN, "chanlock is %lud", *(ulong*)&chanlock);
while(_threadrendezvous(b->tag, 0) == Intred)
;
return 1;
}
buf = altexecbuffered(a, 0);
if(a->op==CHANRCV)
altcopy(me, buf, c->e);
else
altcopy(buf, me, c->e);
unlock(&chanlock);
_procsplx(spl);
return 1;
}